Page 268 - 35_DS702_E_2014_Lightning_Protection_Guide

A frequency converter typically consists of a rectifier, d.c. link, voltage peak occurs that is superimposed on the fundamental
inverter and control electronics (Figure 9.1.1). wave. This voltage peak reaches values of more than 1200 V
At the inverter input, a single-phase a.c. voltage or three- (depending on the frequency converter). The better the simu-
phase line-to-line a.c. voltage is converted into a pulsating d.c. lation of the sinusoidal curve, the better the run and control
voltage and is fed into the d.c. link which also serves as an performance of the motor. This, however, means that voltage
energy storage system (buffer). peaks occur more frequently at the output of the frequency
Capacitors in the d.c. link and earthed L-C sections in the converter.
mains filter can cause problems with upstream residual cur- In order to pick the correct surge arrester for your frequency
rent protective devices (RCDs). These problems are often in- converter, the maximum continuous operating voltage U c
correctly associated with surge arresters. They are, however, must be taken into account which specifies the maximum per-
caused by short-time fault currents of the frequency con- missible operating voltage a surge protective device may be
verter which are sufficiently high to trip sensitive RCDs. This connected to. Owing to the voltage peaks that occur during
can be prevented by using a surge-proof RCD circuit breaker the operation of frequency converters, arresters with a high
which is available with a discharge capacity of 3 kA (8/20 µs) U c value must be used to avoid “artificial ageing” due to the
and higher for a tripping current I ∆n = 30 mA. heating of the surge arrester under “normal” operation condi-
The inverter provides a pulsed output voltage via the control tions and the associated voltage peaks.
electronics. The higher the pulse frequency of the control elec- Heating of surge arresters can lead to a shorter service life and
tronics for pulse width modulation, the more similar is the out- a disconnection of the surge arrester from the installation it is
put voltage to a sinusoidal curve. However, with each pulse a supposed to protect.

INPUT OUTPUT
rectifier d.c. link inverter
+ + motor
L1 V1 V3 V5
U1
M
L2 C V1 3~
W1
L3 – V4 V6 V2 –

control electronics
e.g. 4 – 20 mA
control / monitoring / communication
data

Figure 9.1.1 Basic principle of a frequency converter

large-area shield earthing at both ends
by means of constant force spring of type
SA KRF… (Part No. 919 031 – 919 038)

frequency converter motor
shielded motor
feeder cable
voltage supply connection of the frequency
compact filter converter to the filter

metal mounting plate connected to earth
in general: all lines must be as short as possible

Figure 9.1.2 EMC-compatible shield connection of the motor feeder cable

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